The invention relates to a circuit arrangement for generating clamping pulses for a clamping circuit for the regeneration of the average picture luminance of a video signal.
Clamping circuits are used to superimpose a d.c. voltage on an a.c. voltage signal. In video signals the d.c. voltage portion represents the average picture luminance. Clamping circuits for regenerating the average picture luminance, have been known for a long time already and can be dimensioned according to their use. In a television receiver a simple level diode circuit which keeps the black level constant, satisfies, for example, all the requirements.
When used at the head of a television transmission system the requirements which such a clamping circuit must satisfy are significantly higher, to ensure that the maximum capability of the transmission system can be utilized to the best possible extent. For analog amplifiers this can be the amplifier saturation limit, for digital transmission the characteristic of the A/D converter. In addition, the clamping circuit must be capable of eliminating transient phenomena of preceding transmission systems and superposed low-frequency noise voltages.
German Patent No. 1,809,362 discloses a circuit arrangement which satisfies these requirements. This circuit arrangement operates in accordance with the keyed clamping circuit principal, and locks the signal onto the synchronizing pulse level.
However, as soon as video signals with accompanying sound must be transmitted via such a circuit arrangement in accordance with the Sound-in-Sync(SIS) method, the picture is disturbed to a considerable extent.
These interferences can be reduced to such a level that they are no longer visible, by means of a circuit arrangement described in German Patent application No. 2811221. They are, however, basically still present and maintaining an appropriate signal-to-noise ratio requires a great deal of testing and adjusting efforts and costs.
A known method uses line-frequency clamping of the video signal on the back porch of the horizontal blanking interval, for which a keyed clamping circuit is used. Such a keyed clamping circuit may be formed by a switch and a clamping capacitor. The circuit is switched to the conducting state by each pulse of a clamping pulse signal and as a result thereof the capacitor is charged to the clamping voltage. The clamping pulses can be derived from the leading or trailing edge of the horizontal synchronizing pulse. Deriving the clamping pulses from the leading edge has however the disadvantage that the edges of a sound-in-sync signal transmitted in the synchronizing pulse again trigger clamping pulses during the synchronizing pulse period, which pulses become first active in the picture range and consequently produce significant interferences. Moreover, errors in the vertical blanking interval (V-interval) cannot be completely avoided with circuits of the above type, as the length of the synchronizing bottom lever in the synchronizing pulses and during the V-interval greatly differ from each other.
This might be remedied by making the clamping inoperative during the V-interval, as described in German Patent No. 2162955. If signals with superposed low-frequency noise voltages were present in a circuit as described in this patent, then ramps would be produced in the V-interval in accordance with the superposed interference, which would be modulated into the signal. An additional difficulty for the dimensioning of an optimum clamping circuit is embodied by the accompanying sound in the TV-PCM2 method, which operates with a shortened synchronizing pulse and in which, before and after the color burst, the sound information is contained in the form of PCM pulses. Because of this method it is also no longer possible to derive the clamping pulses from the trailing edge of the synchronizing pulse, which would often be of considerable advantage with regard to the V-interval.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a clamping circuit for the regeneration of the average picture luminance of a video signal, a circuit arrangement for producing clamping pulses, which circuit operates reliably and interference-free, irrespective of the standard (PAL, SECAM or NTSC) of the transmitted video signal and irrespective of whether an additional sound signal in accordance with one of the two methods, sound-in-sync or TV-PCM2, are transmitted at the same time.
According to the invention, this object is accomplished in that line-frequency clock pulses are produced from the video signal pulses with the aid of monostable multivibrators and that the vertical blanking interval is deteted by means of a first counting circuit which is driven by one of the monostable multivibrators. This first counting circuit always selects one of the outputs of a second counting circuit, from which outputs clamping pulse signals are supplied at different instants.